Apparatus and method for separating air by cryogenic distillation

ABSTRACT

An apparatus for separating air, comprising a double column, means for sending air to the purification unit at a pressure that is no more than 1 bar higher than atmospheric pressure, a pipe for sending a first air flow, the first air flow having been purified in the purification unit, to the heat exchanger at a fourth pressure that is no more than 1 bar higher than the second pressure, a pipe for sending the first purified air flow, which has been cooled in the heat exchanger, to the second column for separation, and a booster compressor, the apparatus not comprising any means for depressurizing the first flow.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a § 371 of International PCT Application PCT/FR2019/052934, filed Dec. 5, 2019, which claims the benefit of FR187376, filed Dec. 21, 2018, both of which are herein incorporated by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to an apparatus and to a process for the separation of air by cryogenic distillation.

BACKGROUND OF THE INVENTION

In particular, it relates to an air separation apparatus comprising a double column with a first column operating at a first pressure and a second column operating at a second pressure, lower than the first pressure. The top of the first column produces a gas which condenses in a reboiler of the second column.

It is generally an objective of air separation apparatuses to look for the lowest possible energy consumption.

The purification of air is generally carried out at a pressure equal to or greater than that of the first pressure. This makes it possible to reduce the volume of the purification unit.

It is nevertheless known, from U.S. Pat. No. 4,964,901, to purify a part of the air at the first pressure and the remainder of the air at the second pressure, using two purification units in parallel. The air purified at the second pressure is sent directly to the second column, while the air purified at the first pressure is separated in two, one part being sent directly to the first column and the remainder being boosted, cooled in a heat exchanger, expanded in a turbine coupled to the booster and sent to the second column. Thus the turbine used is a blower turbine and the low pressure column receives air which has been purified at two different pressures.

The process of U.S. Pat. No. 5,934,105 purifies the air at a pressure above the second pressure but below the first pressure; subsequently, the air intended for the first column is compressed and the air intended for the second column is expanded.

JPH11063810 and EP 1 050 730 are similar to U.S. Pat. No. 5,934,105.

If all the flow which goes the second column is expanded in the turbine, as in the prior art, in order to maximize the energy gain, the air flow going to the first column is approximately 66% of the total purified flow, for example in order to produce 96% oxygen. This means that it is necessary to pass 34% of the air flow at a relatively low pressure through the turbine.

SUMMARY OF THE INVENTION

According to the present invention, between 6% and 8% of the air is expanded in an air turbine; thus the turbine according to the prior art is at least 4 to 5 times larger due to the volume flow rate.

As the refrigerating capacity of the process according to the prior art is fixed and remains low since the process does not produce a liquid final product, this means that the expansion ratio of the turbine is very low which gives a turbine which is inefficient and in any case not at all standardized, indeed even nonexistent, among suppliers of cryogenic turbines.

In the case where it is desired to impose the air flow sent to the first column in order to maximize the energy gain, according to the prior art, in operation, the regulation of the refrigerating capacity cannot be done by a reduction in the turbine flow and thus will be done by adjusting the pressure upstream of the turbine, that is to say the purification pressure and ultimately the pressure of the blower. This enormously complicates the regulation and makes it necessary to proportion the purification to the lowest pressure which might be had with a refrigerating capacity lower than nominally expected or in a transient phase. According to certain embodiments of the invention, provision is made for the purification pressure to be very close to the second pressure.

In certain embodiments, the invention provides a process which consumes 1% less energy (2% less if a turbine efficiency reduced by 5% pt is considered) compared to the prior art (for example, according to EP 1 050 730); according to the process of EP 1 050 730, the purification is carried out at a pressure between the first and the second pressure.

The expansion ratio of the process of EP 1 050 730 is low, between 1.2:1 and 3.8:1, preferably between 1.4:1 and 2.5:1, while conventional cryogenic turbines are in an expansion ratio range of between 4:1 and 10:1. The invention uses an expansion ratio which remains at the low limit of this range, thus avoiding having a significantly degraded turbine efficiency.

In EP 1 050 730, the inlet pressure of the purification unit is typically 2.5 bara (instead of approximately 1.3 bara according to certain embodiments of the invention). This process uses a first compressor having several, typically two, stages with cooling between two stages. According to certain embodiments of the invention, the compressor which compresses all the air has a single stage and thus no cooling between two stages.

The apparatus produces a gas flow enriched in oxygen with a particularly low energy.

U.S. Pat. No. 5,666,824 describes a process according to the preamble of claim 1 but in which the first flow is at least partially condensed in an intermediate condenser of the second column. While a gas is formed, it is itself condensed in another intermediate condenser of the second column and the liquid thus formed is sent to the top of the second column. Thus the first flow is not sent directly to the distillation.

WO2013/014252 describes, in FIG. 6, a process in which a first part of the air is cooled to its dew point in a heat exchanger where a flow of air expanded in a turbine is also cooled to its dew point. This is impossible since the waste nitrogen which cools the air flows has already been reheated in a subcooler. In this case, the nitrogen is too hot to cool the air flows to their dew point and the air flows will be cooled at the very most to a temperature approximately 10° C. above the dew point.

Moreover, on calculating the refrigeration balance of FIG. 6, it is found that, by using a compressor upstream of the turbine and by cooling to ambient temperature before expansion, a compression pressure of greater than 80 bar is required. In this case, the expansion ratio of the turbine is much higher than the values used in industry. Thus, it is not possible for a person skilled in the art to implement the method of FIG. 6 as described.

According to a subject matter of the invention, an air separation apparatus is provided comprising a double column with a first column operating at a first pressure and a second column operating at a second pressure, lower than the first pressure, the second column having a bottom reboiler, means for sending nitrogen-enriched gas from the top of the first column to the bottom reboiler and means for sending at least a part of the condensed nitrogen-enriched gas from the bottom reboiler to the top of the first column, a heat exchanger, a purification unit, means for sending air to the purification unit at a third pressure greater than atmospheric pressure by at most 1 bar, a pipe for sending a first flow of air purified in the purification unit to the heat exchanger at a fourth pressure greater than the second pressure by at most 1 bar, a pipe for introducing the first flow of purified air cooled in the heat exchanger into the second column in order to be separated therein, a booster, a pipe for sending a second flow of air purified in the purification unit to the booster, a pipe for sending at least a part of the second flow, compressed by the booster up to a fifth pressure between the first pressure and 1 bar above the first pressure, to the heat exchanger, means for producing refrigeration, a pipe for withdrawing at least one fluid enriched in oxygen or nitrogen from a column of the double column connected to the heat exchanger and a pipe for exiting at least one fluid enriched in oxygen or nitrogen from the heat exchanger as product, the apparatus not comprising any means of expansion of the first flow and comprising only a single purification unit, characterized in that the second column does not comprise an intermediate condenser, the pipe for introducing the first flow of purified air being connected to the inside of the second column in order to make it possible for the first flow to participate in the distillation.

According to other optional aspects:

-   -   the means for the production of refrigeration comprise at least         one turbine for expansion of a part of the second flow and/or         one turbine for expansion of a nitrogen-rich gas originating         from the first column and/or means for sending a cryogenic         liquid from an external source to the double column.     -   the turbine for expansion of the part of the second flow is         connected to the second column in order to send the expanded air         there.     -   the means for sending air to the purification unit at the third         pressure does not comprise any compression means other than a         single-stage compressor.     -   the apparatus does not comprise any means for compression of the         first flow.

According to another aspect of the invention, there is provided a process for the separation of air by cryogenic distillation using a double column with a first column operating at a first pressure and a second column operating at a second pressure, lower than the first pressure, the second column having a bottom reboiler, in which:

-   -   i) air containing water and carbon dioxide is sent to a single         purification unit at a third pressure greater than atmospheric         pressure by at most 1 bar,     -   ii) the purified air is separated into two,     -   ii) a first flow of air purified in the purification unit is         sent to a heat exchanger at a fourth pressure greater than the         second pressure by at most 1 bar,     -   iv) the first flow of purified air cooled in the heat exchanger         is sent to the second column, without having expanded it,     -   v) a second flow of purified air is boosted to a fifth pressure         between the first pressure and 1 bar above the first pressure,         at least a part of the second flow is sent at the fifth pressure         to the heat exchanger and the at least a part of the second flow         is sent to the first column in gaseous form,     -   vi) refrigeration is provided in order to keep the process cold,     -   vii) a nitrogen-rich gas from the first column is at least         partially condensed in the reboiler and at least a part of the         condensed nitrogen is returned to the first column, viii) a         nitrogen-enriched liquid and an oxygen-enriched liquid are sent         from the first column to the second column,     -   ix) an oxygen-enriched gas or a nitrogen-enriched gas is         withdrawn from the double column and it is reheated in the heat         exchanger in order to form a product of the process,         characterized in that the first air flow is sent directly into         the second column in order to be separated therein without         having been condensed in a condenser.

According to other optional aspects:

-   -   the entire first flow is sent to the second column.     -   the first flow is sent to the second column at a level lower         than or equal to the level of arrival of the oxygen-enriched         liquid.     -   the process does not produce any liquid product as final product         and/or no liquid flow is withdrawn from the double column to         serve as final product.     -   the process is kept cold by expansion of a part of the second         flow in a turbine from the fifth pressure to the second         pressure.     -   the part of the air expanded in the turbine represents between 6         vol % and 15 vol %, preferably between 6% and 8%, of the         purified air.     -   all the air is purified at a pressure which does not exceed 1.5         bara, indeed even does not exceed 1.3 bara.     -   all the second flow is cooled in the heat exchanger down to an         intermediate temperature of the heat exchanger, the inlet of the         turbine is at the intermediate temperature of the heat exchanger         and the part of the second flow sent to the first column is         cooled in the heat exchanger down to the cold end of the latter.     -   the first pressure does not exceed 6 bara.     -   the second pressure does not exceed 1.5 bara.     -   the oxygen-enriched gas contains at least 80 mol % oxygen.     -   the oxygen-enriched gas contains at least 90 mol % oxygen.     -   the oxygen-enriched gas contains less than 98 mol % oxygen.     -   the first flow represents between 20 vol % and 30 vol % of the         purified air flow.     -   the second flow represents between 70 vol % and 80 vol % of the         purified air flow.     -   an oxygen-enriched gas and/or a nitrogen-enriched gas is (are)         withdrawn from the double column and it is (they are) reheated         in the heat exchanger in order to form a product of the process         by introducing it or by introducing them at the cold end of the         heat exchanger.     -   the first air flow and/or the part of the second flow intended         for the first column is (are) cooled in the heat exchanger down         to a temperature at least 5° C. above its (their) dew point.     -   an oxygen-enriched liquid is withdrawn and reheated in the heat         exchanger in order to form a product of the process.     -   the oxygen-enriched liquid is pressurized before vaporizing it         either in a dedicated vaporizer or in the heat exchanger.     -   the oxygen-enriched liquid is vaporized by heat exchange with a         part of the second flow or with a third flow of air pressurized         to a pressure greater than the fifth pressure.     -   the first air flow is subcooled between the heat exchanger and         the second column.     -   the part of the air expanded in the turbine is subcooled between         the outlet of the turbine and the second column.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawing(s). It is to be noted, however, that the drawing(s) illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

The FIGURE shows an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described in a more detailed manner with reference to the FIGURE.

FIG. 1 represents a process for the separation of air by cryogenic distillation according to certain embodiments of the invention.

An apparatus for the separation of air by cryogenic distillation comprises a double column with a first column K3 operating at a first pressure and a second column K4 operating at a second pressure, lower than the first pressure, the second column having a bottom reboiler M. The second column K4 does not contain an intermediate condenser.

In this example, the first pressure is 4.5 bara and the second pressure is 1.13 bara.

A nitrogen-enriched gas is sent from the top of the first column to the bottom reboiler M and at least a part of the condensed nitrogen-enriched gas from the bottom reboiler is sent to the top of the first column.

Air at atmospheric pressure is filtered in a filter A, compressed by a blower B having a single stage at a pressure at most 1 bar, preferably at most 0.5 bar, above atmospheric pressure, cooled by a cooling means C and purified of water and carbon dioxide in a single purification unit D in which the air 4 enters at a third pressure greater than atmospheric pressure by at most 1 bar, preferably by at most 0.5 bar. The purification unit comprises two adsorbent beds used alternately to purify the air, one bed purifying the air while the other is regenerated.

The air purified in the unit D is divided into two in order to form two flows 6,8. The air 8 is neither compressed nor expanded and is at a pressure which differs from the second pressure by a pressure equal to the pressure drops in the pipes and the heat exchanger G.

Preferably, the first flow 8 represents between 20 vol % and 30 vol % of the flow 4 and the second flow 6 represents between 70 vol % and 80 vol % of the flow 4.

Thus, the air 8 is sent directly from the purification unit to the second column K2 to be separated therein, entering the column in entirely gaseous form. The air 8 is cooled in the heat exchanger G down to a temperature at least 5° C. above its dew point.

The flow 6 is boosted in a booster E, cooled in a cooler F and sent to the heat exchanger G. The booster E boosts the air 6 up to a fifth pressure between the first pressure and 1 bar above the first pressure. The air 6 is divided into two parts 30,32 at an intermediate level of the exchanger. The air 30 leaves the exchanger at an intermediate temperature of the latter, for example −125° C., is expanded in a turbine 28 down to the second pressure and enters in gaseous form, mixed with the flow 8, to be separated in the second column K4.

The flow 30 can represent between 6 vol % and 15 vol %, preferably between 6% and 8%, of the air 4.

The air 32 is cooled down to the cold end of the exchanger G and is sent to the bottom of the first column K3 in essentially gaseous form in order to be separated therein. The air 8 is cooled in the heat exchanger G down to a temperature at least 5° C. above its dew point.

An oxygen-enriched liquid flow 34 is withdrawn at the bottom of the first column and sent to a level of the second column which is above the air inlet. Alternatively, the air can enter the second column at the same level as that of the arrival of the liquid 34.

The expanded liquid 34 can be separated in a phase separator: the liquid resulting from the phase separator is sent to the column K4 and the vapor phase can be mixed at the inlet of air 8,30 into the column K4.

A flow of liquid nitrogen 35 is withdrawn from the top of the first column and sent to the top of the second column.

Gaseous nitrogen 36 is withdrawn at the top of the second column K4 and is heated in the subcooler S and subsequently in the exchanger G. A part 14 of this gas is used to regenerate the purification unit D.

Gaseous oxygen 29 is withdrawn at the bottom of the second column K4. The flow 29 preferably contains at least 80 mol % oxygen, indeed even at least 90 mol % oxygen, but preferably less than 98 mol % oxygen.

It will be noticed that the process does not produce any liquid flow as final product. The process does not produce any liquid flow to be vaporized in order to form a final gaseous product, optionally under pressure. It is, however, possible to produce a small amount of final gaseous product in this way, which can optionally be mixed with the main gaseous product.

Furthermore, a small flow of liquid might be produced.

In an alternative form, the air 8 and/or the air 30 can be subcooled in the sub cooler S and then be introduced into the second column K4. Otherwise, the mixture of the flows 8 and 30 can be subcooled in the subcooler S and then be introduced into the second column K4.

In the example described, the flow 29 is a flow of gaseous oxygen which is heated in the heat exchanger G from the cold end of the exchanger G. Alternatively, the flow 29 can be a flow of oxygen-rich liquid pressurized to a pressure above that of the second column K4. The liquid 29 is vaporized either in a dedicated vaporizer (not illustrated) or in the heat exchanger G. The liquid 29 can be vaporized by heat exchange with all the air 32 in order to partially condense the air 32, which will subsequently be sent to the bottom of the first column K3. Otherwise the liquid 29 can be vaporized by heat exchange with a part of the air 32 in order to completely condense this part of the air 32. The condensed air will subsequently be sent to the bottom of the first column K3 or to an intermediate point of the first and/or of the second column.

Otherwise, a part of the purified air can be boosted in a booster to a pressure greater than that of the first column K3 in order to vaporize the liquid 29.

While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.

“Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited. 

1-15. (canceled)
 16. An air separation apparatus comprising: a double column with a first column operating at a first pressure and a second column operating at a second pressure, lower than the first pressure, the second column having a bottom reboiler; means for sending nitrogen-enriched gas from the top of the first column to the bottom reboiler and means for sending at least a part of the condensed nitrogen-enriched gas from the bottom reboiler to the top of the first column; a heat exchanger; a purification unit; means for sending air to the purification unit at a third pressure greater than atmospheric pressure by at most 1 bar; a pipe for sending a first flow of air purified in the purification unit to the heat exchanger at a fourth pressure greater than the second pressure by at most 1 bar; a pipe for introducing the first flow of purified air cooled in the heat exchanger into the second column in order to be separated therein, a booster; a pipe for sending a second flow of air purified in the purification unit to the booster; a pipe for sending at least a part of the second flow, compressed by the booster up to a fifth pressure between the first pressure and 1 bar above the first pressure, to the heat exchanger; means for producing refrigeration; a pipe for withdrawing at least one fluid enriched in oxygen or nitrogen from a column of the double column connected to the heat exchanger and a pipe for exiting at least one fluid enriched in oxygen or nitrogen from the heat exchanger as product; and the apparatus not comprising any means of expansion of the first flow and comprising only a single purification unit; wherein the second column comprises an absence of an intermediate condenser, wherein the pipe for introducing the first flow of purified air is connected to the inside of the second column such that the first flow is included in the distillation.
 17. The apparatus as claimed in claim 16, in which the means for the production of refrigeration includes at least one of: a turbine for expansion of a part of the second flow, a turbine for expansion of a nitrogen-rich gas originating from the first column, and means for sending a cryogenic liquid from an external source to the double column.
 18. The apparatus as claimed in claim 17, in which the turbine for expansion of the part of the second flow is connected to the second column in order to send the expanded air to the second column.
 19. The apparatus as claimed in claim 16, in which the means for sending air to the purification unit at the third pressure comprises an absence of any compression means other than a single-stage compressor.
 20. The apparatus as claimed in claim 16, further comprising an absence of any means for compression of the first flow.
 21. A process for the separation of air by cryogenic distillation using a double column with a first column operating at a first pressure and a second column operating at a second pressure, lower than the first pressure, the second column having a bottom reboiler, in which: a. sending air containing water and carbon dioxide to a single purification unit at a third pressure greater than atmospheric pressure by at most 1 bar; b. separating the purified air into two; c. sending a first flow of air purified in the purification unit to a heat exchanger at a fourth pressure greater than the second pressure by at most 1 bar; d. sending the first flow of purified air cooled in the heat exchanger to the second column, without having expanded the first flow of purified air; e. boosting a second flow of purified air to a fifth pressure between the first pressure and 1 bar above the first pressure, and then sending at least a part of the second flow at the fifth pressure to the heat exchanger and sending the at least a part of the second flow to the first column in gaseous form; f. providing refrigeration; g. at least partially condensing a nitrogen-rich gas from the first column in the reboiler and returning at least a part of the condensed nitrogen to the first column; h. sending a nitrogen-enriched liquid and an oxygen-enriched liquid from the first column to the second column; and i. withdrawing an oxygen-enriched gas or a nitrogen-enriched gas from the double column and reheating said oxygen-enriched gas or said nitrogen-enriched gas in the heat exchanger in order to form a product of the process, wherein the first air flow is sent directly into the second column in order to be separated therein without having been condensed in a condenser.
 22. The process as claimed in claim 21, in which the first flow is sent to the second column at a level lower than or equal to the level of arrival of the oxygen-enriched liquid.
 23. The process as claimed in claim 21, kept cold by expansion of a part of the second flow in a turbine from the fifth pressure to the second pressure, the part of the air expanded in the turbine preferably representing between 6 vol % and 15 vol %, preferably between 6% and 8%, of the purified air.
 24. The process as claimed in claim 23, in which all the second flow is cooled in the heat exchanger down to an intermediate temperature of the heat exchanger, the inlet of the turbine is at the intermediate temperature of the heat exchanger and the part of the second flow sent to the first column is cooled in the heat exchanger down to the cold end of the latter.
 25. The process as claimed in claim 21, in which all the air is purified at a pressure which does not exceed 1.5 bara, indeed even does not exceed 1.3 bara.
 26. The process as claimed in claim 21, in which the oxygen-enriched gas contains at least 80 mol % oxygen, indeed even at least 90 mol % oxygen, but preferably less than 98 mol % oxygen.
 27. The process as claimed in claim 21, in which the first flow represents between 20 vol % and 30 vol % of the purified air flow.
 28. The process as claimed in claim 21, in which the second flow represents between 70 vol % and 80 vol % of the purified air flow.
 29. The process as claimed in claim 21, in which an oxygen-enriched gas and/or a nitrogen-enriched gas is withdrawn from the double column and it is reheated in the heat exchanger in order to form a product of the process by introducing it or by introducing them at the cold end of the heat exchanger.
 30. The process as claimed in claim 21, in which the first air flow and/or the part of the second flow intended for the first column is cooled in the heat exchanger down to a temperature at least 5° C. above its dew point. 